The bug comes into play if your weapons or attacks are too powerful. Defenses don't make it come into play just power attacks. So if you play as a barbarian Nord, try using low level archery or destruction magic. This stops the jump from 20+% to 0 in one stroke. If that happens he becomes invunerable. Also if you have a lot of jewellry that boosts fire, cold, electricity etc effects, unequip them as well. Then when he drops tp lets say 17%, he calls for a dragon, absorbs the soul, now only 2 to go.